Calcium plays a significant role in various cellular processes including cell signaling or membrane fusion. Calcium ions have strong interactions with the negatively charged inner leaflet of plasma membrane (PM) where are the target molecules of many signaling pathways. Changes in biophysical properties of phospholipid membranes upon calcium binding were recently investigated (1). We took another step in further understanding the calcium binding to biologically relevant model of PM by incorporating a transmembrane peptide. We designed the peptide as a single transmembrane helix with either zero total charge or with a mild amount of positive charge. The positively charged peptide follows the so called positive-inside rule stating that the excess positive charge of helical TM proteins resides at the cytoplasmic side of the PM. The positive charge is however expected to repel the calcium ions. We use time-resolved emission spectroscopy of lipid vesicles accompanied by zeta potential measurements. The fluorescence technique gives us information on hydration and mobility of molecules in the vicinity of a fluorescent probe whereas the zeta potential shows trends in surface charge on the studied membranes. We found out that both transmembrane peptides indeed rigidify phospholipid bilayers. The positive charge only strenghtens this effect. Surprisingly, the positive charge of the peptide has no influence on calcium binding to membranes which is the same regardless the peptide content.

1. Melcrova et al. The complex nature of calcium cation interactions with phospholipid bilayers. Scientific Reports. 2016;6(1):38035. Available from: doi: 10.1038/srep38035.

Financing: Czech Science Foundation, Martina Roeselová Memorial Fellowship, Charles University project SVV